Cone penetrometer testing (CPT) is commonly used for the geotechnical and environmental characterization of subterranean media. Some aspects of this characterization require the analysis of samples of soil retrieved from subterranean layers. Appropriately used, CPT has become a cost effective alternative to other methods for obtaining precise stratigraphic and chemical information to depths over 300 feet below the surface of the ground.
A CPT system typically involves the deployment of a cylindrical probe through subterranean media at a constant velocity specified by industrial standards. Sensors are housed in the cylindrical probe having a conical point that is pushed vertically without rotation into the ground via hydraulic or other constant velocity ram systems. As the probe advances, additional cylindrical tubes or rods are added to a string of tubes or rods to increase the effective length of the probe.
The vertical load exerted on the probe as it is advanced can exceed thirty tons of force. This force is preferably developed by the dead weight of the vehicle to which the ram system is attached or alternatively by a reaction against earth anchors.
The lower end of the CPT probe, approximately 1.5 inches in diameter, contains load sensors for measuring the vertical bearing load on the conical surface section of the probe and the vertical frictional or shear load on the external surface of a short cylindrical section disposed immediately above the conical surface section. These two loads are measured at multiple intervals of depth or time to produce a continuous representation of various geomechanical soil properties as a function of depth. In addition, empirical correlations are commonly used to develop stratigraphic maps from these load measurements.
Optionally, additional sensors are deployed within the probe to measure the pressure of the pore fluid, electrical resistance and moisture content of the soil matrix, alkalinity and oxidation reduction potential of the subterranean media, and seismic velocity response to an imposed surface wave to derive soil bulk modulus. After the desired depth or maximum advisable push load is obtained, the probe and connecting rods are typically removed from the ground.
Because of the efficiency of the CPT device push method, CPT push systems have been adapted to push other types of devices into the ground. Specifically relevant to this invention, these devices often include material sampling tools. Several devices have been previously developed for obtaining samples of soil, pore fluid and pore vapor. Desirable qualities of such tools should be that they withstand the extreme normal and shear loads applied to CPT-deployed probes during the push and retrieval processes, that they reliably obtain a quality sample of media from the area of interest, that they retain the sample during retrieval, and that they are easy to unload and redeploy without cross-contaminating additional samples.
Several attempts have been made to develop a soil sampler that embodies these qualities. Those designed to be opened after being pushed to the desired sampling depth have previously utilized two tip release devices and methods.
The first such device consists of load-bearing keys disposed between a movable tip assembly and an outer housing. The keys are displaced by inertial means when the housing is pulled upward with respect to the tip assembly. With the keys displaced, the housing can be driven further into the soil, thereby moving the housing relative to the movable tip assembly.
The second device releases a similar movable tip assembly by means of a lanyard lowered through an inner diameter of the cone rods, once the sampler has been pushed to a desired depth. The tip assembly includes hardened steel balls projecting radially outward into an inner radial groove within the housing. The lanyard engages and is secured to an upper portion of the tip assembly, which when displaced upward with respect to a lower portion of the tip assembly allows the steal balls to move radially inward out of the groove, thus releasing the tip assembly. Further upward motion of the lanyard pulls the tip assembly, both upper and lower sections, through the outer housing until reaching the upper end of the sampler.
Upon reaching the upper end of the sampler, an appropriate device fixed to the housing disengages the lanyard from the tip assembly to facilitate the removal of the lanyard prior to the retrieval of the sampler. The present invention improves upon the second device heretofore described.
Other devices exist which have been improved upon by the development of the devices described above. Such devices include samplers which do not provide a sealing means between the tip assembly and the outer housing prior to tip release and samplers which do not provide sufficient sample volume or reliable means for retaining the sample during retrieval.
The above-described soil sampling devices have several shortcomings.
The device of the first type described, using locking keys, relies upon inertial forces to release the locking mechanism between the tip and the housing. Such inertial forces presume a vertical push direction, parallel to normal gravitational forces. However, certain circumstances require pushing at an acute angle to vertical, such as when attempting to secure a sample underneath an object, for example, a storage tank, in which case the locking keys do not reliably disengage. Even during vertical pushes the keys may fail to disengage, at which point the operator has no indication that a failure has occurred until the sampler is retrieved.
Additionally, the release method requires the keys to be separate objects which must be manually set in place when assembling the sampler between each sampling operation. The use of appropriate environmental field apparel, such as work gloves, impedes the installation of such individual keys and makes field assembly difficult. Further, the release method of pulling upward on the outer housing disallows a common procedure of repeatedly displacing the rods up and down a few inches to work the probe through thin hard layers of soil. This procedure, commonly called "cycling", causes the tip assembly of this type of soil sampler to prematurely release before reaching a desired depth.
In addition, samplers of this type do not reliably retrieve samples from weak saturated soil layers because the weight of the released tip assembly bears upon the soil to be sampled as the housing is advanced. This weight prevents the soil from entering the sample chamber as required, as even small normal forces exacerbate the phenomenon of granular arching at the opening of the sample tube. Non-cohesive soils also present a difficulty to this tip release mechanism, as there must be sufficient retention of the conical tip surface in the soil to keep the tip from moving upward when the housing is raised to release the locking keys. Non-cohesive sandy soils, therefore, often keep the tip from releasing as required.
While attempting to address some of the above-described shortcomings by providing a means to actively release the locking mechanism, pull the released tip assembly through the sample chamber and retain it at the top of the chamber, prior examples of samplers of the second type described have had their own shortcomings. For example, the load bearing area, between the locking steel balls and the mating grooved surface, has not been sufficient to withstand the contact forces developed by pushing the sampler in harder geologic materials. The result in several instances is that the housing member containing the grooved surface will fracture during pushing. Another result of the high contact loads is that the grooved surface will deform with repeated loading and will have to be replaced.
The mechanical arrangement of balls in a groove is common to the design of radial ball bearings and works well for large radial loads combined with minor axial loads. The nature of this application to soil samplers requires the mechanism to withstand relatively high axial loads and almost no radial load.
Attempts to remove this limitation in current designs have been largely unsuccessful as they have compromised ease of use. In addition, samplers of this type have been unable to reliably obtain samples of loose, granular material which flow back out of the sample chamber during retrieval. Further, decontaminating the tip assemblies of such samplers has proven difficult due to the large number of individual members comprising the tip assembly and numerous small crevices which retain sample media.